A recent study found that residents of Canadian communities who were exposed to emissions from polluting industries such as oil refineries, metal smelters, and pulp mills gave birth to more females than males, a reversal of the normal sex ratio. This is likely due to high levels of common air pollutants called dioxins and is not a surprising finding, according to James Argo, a medical geographer with the IntrAmericas Centre for Environment and Health, who conducted the study. “There is a very strong association [in the scientific literature] between chronic exposure to dioxins and an inverted sex ratio,” he said.

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The study is the second phase of a three-part project to examine the links between early exposure to industrial pollution and the development of cancer. In the early 1990s, Argo documented the lifetime residences of 20,000 people who had cancer and 5,000 “control” subjects who did not have the illness. The database was developed to inform research about people’s exposure to industrial pollutants throughout their lifetimes, including prenatal exposure, Argo said.

The inverted sex ratios became apparent when Argo looked at the genders of children born to parents who lived within 25 kilometers of a polluting industry. The percentage of girls was higher in all of the nearly 90 communities surveyed, and in some communities, residents gave birth to as few as 46 males for every 54 females, compared to a normal sex ratio of 51 males for every 49 females. Chronic exposure to dioxins “interferes with the process of conception,” so people who have been exposed for over 20 years or so “will have a higher probability of giving birth to females,” Argo said.

Studies conducted in Russia, Italy, and elsewhere have also linked inverted sex ratios to dioxin exposure, but these have tended to focus on exposure in the workplace, not in the wider community. Argo’s research suggests that the influence of the pollutants is more far reaching than was previously thought. “[This] may represent one of only a few studies explicitly designed to identify the impact of carcinogens from industrial sources on residents at home,” he explained. To conduct the analysis, Argo used data from the 1991 Canadian Census and combined it with his own data collected between 1993 and 1995.

The third phase of the project will use Argo’s latest findings on inverted sex ratios to examine the documented rise in female reproductive cancers. With more females than males in a population, there is likely to be a greater incidence of breast, uterine, ovarian, and cervical cancers in that group, he explained. But male reproductive health problems are also on the rise, he noted.